Using criteria in 16 categories, including air and water quality, traffic congestion, access to public transportation, land use, green business activity, and environmental policies, SustainLane, an Internet company that focuses on health and environmental issues, ranks Portland as the greenest of the 50 largest US cities. The city, which was once known as "Stumptown" because of the way its land was quickly logged, has ranked first since the company began compiling the lists in 2005.

The bottom line: The kind of things that make cities “sustainable” also make them expensive. Take the rankings of “housing affordability.” The cheapest cities—San Antonio, Fort Worth, Arlington, El Paso—also scored the worst on public transit, bike-friendliness, and ability to walk to work. The big winners there are also among the most expensive places to live, like San Francisco, New York, San Jose, and Boston.

Then again, with gas prices around $4 a gallon, it's not getting any cheaper to live in cities without decent bus and rail systems.

James Elsen, SustainLane's founder and CEO, notes that in the past the greenest cities have had something else in common: they're full of liberals. And the least green of the 50 largest cities are packed with conservatives. But, as Mr. Elsen noted in May in the online eco-mag Grist, this distinction is fast blurring:

In that first ranking, released in spring 2005, we found that Portland came out on top, with San Francisco and Seattle not far behind. We learned that West Coast cities and "blue" cities (New York, Chicago, Boston) seemed to be greener than "red" ones. We chalked that up, in part, to the notion that modern sustainability grew out of 1970s environmentalism – often the domain of Democrats, while Republicans saw environmentalism as anti-economic growth back then.